1. Field of the Invention
The present invention relates to glass compositions suitable for covering electrodes, and display panels, particularly plasma display panels, using the same.
2. Related Background Art
In displays and integrated circuits such as plasma display panels (hereinafter abbreviated as “PDP”), field emission displays, liquid crystal displays, fluorescent displays, ceramic laminated devices, and hybrid integrated circuits, substrates are used that have electrodes and wirings formed of, for example, Ag or Cu on the surfaces thereof. Such electrodes and wirings may be covered with insulating glass materials to be protected. A PDP, which is a typical display, is described below using an example.
Generally, a PDP is configured to include two opposing glass substrates, each of which is provided with electrodes arranged regularly, with gas that mainly contains inactive gas, such as Ne or Xe, being sealed therebetween. In the PDP, voltage is applied between electrodes to cause electric discharge in minute cells located around the electrodes and to allow each cell to emit light, resulting in a display. These electrodes are covered with an insulating material called a dielectric layer to be protected.
For instance, in a glass substrate to serve as the front plate of an AC type PDP, transparent electrodes are formed and electrodes of metal, such as Ag, Cu, or Al, with lower resistivity are formed further thereon. A dielectric layer is formed covering those combined electrodes, and a protective layer (MgO layer) is formed further thereon.
The dielectric layer to be formed covering the electrodes can be a thin film of, for example, SiO2 formed by a method such as CVD (chemical vapor deposition). Usually, however, from the viewpoints of equipment and cost, glass with a low softening point is used. The dielectric layer made using such glass with a low softening point is formed by applying a paste containing glass powder to cover electrodes by, for instance, a screen printing method or a die coating method, and then baking it.
The characteristics required of a glass composition that forms a dielectric layer include, for example:
(1) having insulation properties because it is formed on electrodes;
(2) having a thermal expansion coefficient that is not greatly different from that of the substrate material so as to prevent the glass substrate from warping and the dielectric layer from peeling off and cracking in the case of a large-area panel;
(3) being amorphous glass with a high visible light transmittance so as to utilize the light generated from phosphors as display light efficiently when being used for a front panel; and
(4) having a lower softening point so as to conform to the heat resistance of substrate glass.
Examples of the glass substrate to be used for a PDP include soda lime glass, which is window sheet glass that is produced by a float process and generally is easily available, and glass with a high strain point developed for PDPs. They usually have a heat resistance up to 600° C. and a thermal expansion coefficient of 75×10−7 to 85×10−7/° C.
Accordingly, with respect to the item (2) described above, the glass composition has desirably a thermal expansion coefficient of about 70×10−7/° C. to 90×10−7/° C. With respect to the item (4) described above, since it is necessary to bake the glass paste below 600° C., which is the strain point of the glass substrate, the glass composition desirably has a softening point of about 590° C. or lower so as to be softened enough even if the glass paste is baked at a temperature of 600° C. or lower.
Currently, PbO—SiO2 glass whose main raw material is PbO is used mainly as a glass material that satisfies the requirements as described above.
However, environmental concerns in recent years require dielectric layers that are free from Pb. Furthermore, glass materials are required to have further lower permittivity in order to reduce the power consumption of PDPs. For example, a Bi2O3—B2O3—ZnO—SiO2 glass material (see, for example, JP 2001-139345 A) having a lower softening point achieved by providing zinc borate as a main component and Bi instead of Pb has been developed as glass that is free from Pb. However, like the Pb material, the Bi material also has a problem in that its relative permittivity is as high as about 9 to 13.
Therefore, a material that attained a relative permittivity of around 7 also is proposed using the zinc borate glass (alkali metal oxide-B2O3—ZnO—SiO2 glass) containing alkali metal instead of Pb in order to obtain both a low permittivity and a low softening point (see, for example, JP 9(1997)-278482 A, JP 2000-313635 A, and JP 2002-274883 A).
However, although the desirable relative permittivity is preferably as low as possible, glass with a relative permittivity of 6.5 or lower and a low softening point had not been found. Furthermore, the alkali zinc borate glass that has been studied conventionally can satisfy the low softening point and suitable thermal expansion coefficient but it has been difficult to obtain glass with a high glass transition temperature (glass transition point) in addition to the low softening point and suitable thermal expansion coefficient.
If the glass to be obtained is one for covering electrodes simply, it is enough to allow it to have a low softening point, a suitable thermal expansion coefficient, and a low permittivity. However, in the case of PDPs, after electrodes are covered with glass, the glass layer is heated again at a temperature of nearly 500° C. in, for example, the step of annealing an MgO layer and the sealing step of bonding a front panel and a rear panel to each other. Since the softening point of the glass for a dielectric layer is a little lower than 600° C., it will not necessarily be softened even if it is heated at a temperature of about 500° C. However, if this heating temperature exceeds the glass transition temperature considerably, the physical properties of the glass will change rapidly. Accordingly, especially, in large area displays, a dielectric layer will separate from a substrate or will crack, which results in a decrease in insulation and reliability. According to the studies made by the inventor, in order to reheat-treat the glass at about 500° C., the desirable glass transition point required of the glass is at least 475° C. Moreover, for example, in displays other than PDPs and circuit boards, when electrodes and wirings are covered with a glass material and thereafter the glass material is heat-treated at a high temperature again, there was a risk of similar problems arising.
According to the studies of the inventor, in order to allow alkali zinc borate glass to have a low permittivity, it is necessary to increase the amount of B2O3, but an increase in the amount of B2O3 tends to lower the glass transition point. In the conventional glass for covering electrodes, attention was not paid to the glass transition point at all. Accordingly, although the material with a low softening point, a low permittivity, and a suitable thermal expansion coefficient is obtained, no material that also has a high glass transition point in addition to them has been obtained.
Furthermore, in alkali glass with a large content of B2O3, since B2O3 dissolves in water easily, there was a problem that it has low water resistance/high hygroscopicity. Low water resistance may result in insulation failure from exposure of the glass of which a dielectric layer is formed to water during cutting of the substrate. On the other hand, the high hygroscopicity may increase the amount of moisture in the system (in the dielectric layer), deteriorating the properties of the MgO protective film formed on the dielectric layer.